Let me respond.
1. Being a former VP is big, but the rules of politics have changed. Having a connection to a former popular president doesn't guarantee you the win, or else Gore and Clinton would have gotten the WH. But what's more, almost every candidate who previously failed to win the primary of their party ended up losing the general election, if and when they did win it down the road. Doyle, Gore, Clinton, Romney, and McCain all attest to that history.
2. Clinton was also popular with the black community because of her legacy with Big Bill, until somebody else sounded better. Yes, Obama had the concept of being the first black president, but people knew that for a long time before they ditched Clinton, so that's not all there was to it. It was her flawed record and being out of step with where the base was.
3. Biden leans liberal at best, and conservative worst. He is by no means center-left. I agree that those kinds of candidates typically get the nod, but these are new times.
Biden can indeed win and the cards are in his hands, but the games has changed a lot, and his platform of "I'm the most electable, because I'm the most electable!" and his wife basically conceding he doesn't inspire anybody, really speaks to the reality of how shaky the foundation on which his house is built.